In these years, due to development of downsizing technologies in a semiconductor field, it has become possible to form a large power device such as a power transistor, a free-wheeling diode and so on, and a CMOS logic circuit on one chip in a composite manner. It also has become possible to downsize overwhelmingly as compared with a conventional voltage control device of a hybrid IC type which is fabricated by disposing discrete devices on a substrate.
Such a CMOS digital IC is suitable for speeding up operational speed and stabilization of a controlled state, and is also very small in size. It therefore excels at mounting capability to a power generator and so on. However, since a logic state is unstable at the time of power-on for starting an operation, it is necessary to inevitably reset and initialize. Also, for transistors etc. which are included in such an IC, an initial minimum voltage for compensating for an operation is determined. Further, in order for each constituent circuit (e.g., a power supply circuit for generating an operational voltage which is supplied to each circuit) in a power generation control device to operate normally, it is necessary to apply a voltage with a predetermined value or above.
On the other hand, various electric loads are connected to a power system of a vehicle. Due to power-on, power-off and so on to these electric loads, there frequently occur voltage fluctuations in which an output voltage of a power generator and a terminal voltage of a battery vary. Also, computerization has been advanced in various kinds of electric loads. The latest computerized devices are in such an environment that high frequency noises tend to take place at the time of power-on and power-off.
Also, in recent days, it is determined that, by reducing power generation torque of a power generator, idling rotation of an engine is stabilized. For the purpose of improving a fuel cost and of reducing exhaust gas and so on, cooperative control for intentionally suppressing power generation is carried out by sending a power generation suppression signal from an engine control device etc. to a voltage control device as disclosed in U.S. Pat. No. 5,231,344 (JP2651030, pages 3–9, FIGS. 1–29) and JP-A-8-266097, pages 3–6, FIGS. 1–6.
In these publications, at the time of suppressing power generation, a power operation point is lowered. However, under this state, when noise, in particular, a negative surge current is overlaid or superimposed on a power cable of a battery, a voltage to be applied from the power generator to the voltage control device is further lowered, which is evidenced by the study of the present inventors and so on. As a result of this, the voltage control device temporarily enters an inoperative state along with lowering of the output voltage of the power generator, and a power control operation becomes unstable. When the output voltage of the power generator rises and the voltage control device returns again to the operative state, it is repeated from a reset operation. Therefore, a certain period of time is required until it comes back to a stable voltage control state.
It also has become clear that, in case that a capacitor is disposed in addition, particularly, in the inside of an inductive electric load, for example, in a power system of various motors etc., a resonance phenomenon occurs at the time of transition of power-on by inductance of the load and the capacitor. This resonance causes negative surge currents resulting in noise generation.
It is possible to reduce noises which are generated due to power-on etc., by for example, disposing a capacitor, a ferromagnetic core and so on in the voltage control device. In order to obtain effective large capacity, a film capacitor, an electrolytic capacitor and so on are required. In respect to durability and downsizing, it is difficult to be adopted. Also, the ferromagnetic core is not advantageous in respect to cost, man-hours for building, and a temperature characteristic, and it is also difficult to be adopted.